Lighting device for headlights with a phase modulator

ABSTRACT

There is provided a lighting device arranged to produce a controllable light beam for illuminating a scene. The device comprises an addressable spatial light modulator arranged to provide a selectable phase delay distribution to a beam of incident light. The device further comprises fourier optics arranged to receive phase-modulated light from the spatial light modulator and form a light distribution. The device further comprises projection optics arranged to project the light distribution to form a pattern of illumination as said controllable light beam.

FIELD OF THE DISCLOSURE The present disclosure relates to the field ofillumination. Examples disclosed herein generally relate to a vehicleheadlamp using a spatial light modulator. BACKGROUND

Light scattered from an object contains both amplitude and phaseinformation. This amplitude and phase information can be captured on,for example, a photosensitive plate by well known interferencetechniques to form a holographic recording, or “hologram”, comprisinginterference fringes. The “hologram” may be reconstructed byilluminating it with suitable light to form a holographicreconstruction, or image, representative of an original object, forexample.

It has been found that a holographic reconstruction of acceptablequality for viewing an image of the object can be formed from a“hologram” containing only phase information related to the originalobject. Such holographic recordings may be referred to as phase-onlyholograms.

The term “hologram” therefore relates to the recording which containsinformation about an object and can be used to form a reconstruction.The hologram may contain information in the frequency, or Fourier,domain.

Computer-generated holography may numerically simulate the interferenceprocess, using Fourier techniques for example, to produce acomputer-generated phase-only hologram. A computer-generated phase-onlyhologram may be used to produce a holographic reconstruction.

It has been proposed to use holographic techniques in an illuminationsystem. The system may accept a temporal sequence of 2D illuminationdistributions as an input. The input may be converted into a real-timesequence of corresponding holograms (for example, phase-only holograms)wherein each hologram corresponds to one image frame. The holograms maybe reconstructed in real-time to produce a 2D projection representativeof the input. Accordingly, there may be provided a real-time 2Dprojector to project a sequence of frames using a sequence ofcomputer-generated holograms.

An advantage of projecting video images or light distributions viaphase-only holograms is the ability to control many reconstructionattributes via the computation method—e.g. the aspect ratio, resolution,contrast and dynamic range of the projected light. A further advantageof phase-only holograms is that substantially no optical energy is lostby way of amplitude modulation.

A computer-generated phase-only hologram may be “pixellated”. That is,the phase only hologram may be represented on an array of discrete phaseelements. Each discrete element may be referred to as a “pixel”. Eachpixel may act as a light modulating element such as a phase modulatingelement. A computer-generated phase-only hologram may therefore berepresented on an array of phase modulating elements such as a liquidcrystal on silicon (LCOS) spatial light modulator (SLM). The LCOS may bereflective meaning that modulated light is output from the LCOS inreflection.

Each phase modulating element, or pixel, may vary in state to provide acontrollable phase delay to light incident on that phase modulatingelement. An array of phase modulating elements, such as a LCOS SLM, maytherefore represent (or “display”) a computationally determinedphase-delay distribution. If the light incident on the array of phasemodulating elements is coherent, the light will be modulated with theholographic information, or hologram. The holographic information may bein the frequency, or Fourier, domain.

Alternatively, the phase-delay distribution may be recorded on akinoform. The word “kinoform” may be used generically to refer to aphase-only holographic recording, or hologram.

The phase delay may be quantised. That is, each pixel may be set at oneof a discrete number of phase levels.

The phase-delay distribution may be applied to an incident light wave byilluminating the LCOS SLM, for example. The position of thereconstruction in space may be controlled by using a optical Fouriertransform lens, to form a holographic reconstruction, or “image”, in thespatial domain.

A computer-generated hologram may be calculated in a number of ways,including using algorithms such as Gerchberg-Saxton. TheGerchberg-Saxton algorithm may be used to derive phase information inthe Fourier domain from amplitude information in the spatial domain(such as a 2D image or light distribution). That is, phase informationrelated to the object may be “retrieved” from intensity, or amplitude,only information in the spatial domain. Accordingly, a phase-onlyholographic representation of an object in the Fourier domain may becalculated.

The holographic reconstruction may be formed by illuminating the Fourierdomain hologram and performing an optical Fourier transform using aFourier transform lens, for example, to form an image (holographicreconstruction) at a reply field such as on a screen.

FIG. 1 shows an example of using a reflective SLM, such as a LCOS, toproduce a holographic reconstruction at a replay field location.

A light source (110), for example a laser or laser diode, is disposed toilluminate the SLM (140) via a collimating lens (111). The collimatinglens causes a generally planar wavefront of light to become incident onthe SLM. The direction of the wavefront is slightly off-normal (i.e. twoor three degrees away from being truly orthogonal to the plane of thetransparent layer). The arrangement is such that light from the lightsource is reflected off a mirrored rear surface of the SLM and interactswith a phase-modulating layer to form an exiting wavefront (112). Theexiting wavefront (112) is applied to optics including a Fouriertransform lens (120), having its focus at a screen (125).

The Fourier transform lens receives (phase modulated) light from the SLMand performs a frequency-space transformation to produce a holographicreconstruction at the screen (125) in the spatial domain.

In this process, the light from the light source is generally evenlydistributed across the SLM (140), and across the phase modulating layer(array of phase modulating elements). Light exiting the phase-modulatinglayer may be distributed across the screen. There is no correspondencebetween a specific image region of the screen and any onephase-modulating element.

The Gerchberg Saxton algorithm considers the phase retrieval problemwhen intensity cross-sections of a light beam, I_(A)(x,y) andI_(B)(x,y), in the planes A and B respectively, are known and I_(A)(x,y)and I_(B)(x,y) are related by a single Fourier transform. With the givenintensity cross-sections, an approximation to the phase distribution inthe planes A and B, Φ_(A)(x,y) and Φ_(B)(x,y) respectively, is found.The Gerchberg-Saxton algorithm finds good solutions to this problem byfollowing an iterative process.

The Gerchberg-Saxton algorithm iteratively applies spatial and spectralconstraints while repeatedly transferring a data set (amplitude andphase), representative of I_(A)(x,y) and I_(B)(x,y), between the spatialdomain and the Fourier (spectral) domain. The spatial and spectralconstraints are I_(A)(x,y) and I_(B)(x,y) respectively. The constraintsin either the spatial or spectral domain are imposed upon the amplitudeof the data set. The corresponding phase information is retrievedthrough a series of iterations.

There have been disclosed various techniques for providing improved 2Dimage projection systems using a computer-generated hologram and thesemay also be applied to light distributions used in illuminationapplications.

SUMMARY

In one aspect dynamically varying holograms are used to providecontrolled illumination of a desired part of a scene.

In another aspect there is provided a method of illuminating a scenecomprising forming a varying set of phase distributions on a spatiallight modulator, illuminating the spatial light modulator to provide anexit beam, applying the exit beam to Fourier optics to form an image andprojecting the image to provide a scene-illuminating beam

The method may comprise selecting the set to steer the sceneilluminating beam

The method may comprise selecting the set to illuminate chosen areas ofthe scene while not illuminating other areas.

The method may comprise reading pre-calculated phase distributions froma memory.

The spatial light modulator may be an LCOS SLM.

In another aspect, there is provided a lighting device arranged toproduce a controllable light beam for illuminating a scene, the devicecomprising: an addressable spatial light modulator arranged to provide aset of selectable phase delay distributions to a beam of incident light;Fourier optics arranged to receive phase-modulated light from thespatial light modulator and form an image; and projection opticsarranged to project the image to form a pattern of illumination as saidcontrollable light beam.

The Fourier lens is arranged to form a light distribution at a replayfield which in some cases may be considered an “image”. It can beunderstood from the present disclosure that the light distribution atthe replay field of the Fourier lens may not a real image in space.

In summary, the present disclosure relates to using a computer-generatedphase-only hologram to produce a light distribution for an illuminationdevice such as a headlamp. The holographic reconstruction (orreconstructed image) is projected using projection optics to produce alight distribution in space. For example, the holographic reconstructionmay be projected on a road, for example, to produce a light distributionsuitable for night time driving. Since the computer-generated hologrammay be readily or quickly changed, the projected light distribution maybe dynamically changed. For example, the projected light distributionmay be moved or steered. The system may therefore be incorporated intothe headlights of a motor vehicle to provide controllable illumination.The computer-generated hologram may be changed in real-time to providedynamically varying illumination for a driver, for example. Thecomputer-generated hologram may be changed in response to road ordriving conditions, for example.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic showing a reflective SLM, such as a LCOS, producea holographic reconstruction at a replay field location;

FIG. 2 is a chart showing the function of a modified Gerchberg-Saxtonalgorithm;

FIG. 3 shows an example random phase seed for the first iteration of thealgorithm of FIG. 2;

FIG. 4 shows an illumination system;

FIG. 5 is a schematic of a LCOS SLM;

FIG. 6 is a flow chart of an embodiment for dynamically modifying a headlamp illumination profile in view of an approaching vehicle;

FIG. 7 shows an example system in accordance with the present disclosurefor illuminating a road sign;

FIG. 8 is a flow chart of an embodiment for dynamic road illumination;and

FIG. 9 shows an example infra-red grid system in accordance withembodiments.

In the figures, like reference numerals referred to like parts.

DETAILED DESCRIPTION OF THE DRAWINGS

Holographically generated 2D images are known to possess significantadvantages over their conventionally projected counterparts, especiallyin terms of definition and efficiency. However, the computational andhardware complexity of the current hologram generation algorithmspreviously precluded their use in real-time applications. Recently theseproblems have been solved—see, for example, published PCT application WO2005/059881 incorporated herein by reference.

Modified algorithms based on Gerchberg-Saxton have been developed—see,for example, co-pending published PCT application WO 2007/131650incorporated herein by reference.

These improved techniques are able to calculate holograms at asufficient speed that 2D video projection is realised. Examplesdescribed herein relate to projection using a computer-generatedhologram calculated using such a modified Gerchberg-Saxton algorithm.

FIG. 2 shows a modified algorithm which retrieves the phase informationψ[x,y] of the Fourier transform of the data set which gives rise to aknown amplitude information T[x,y] 362. Amplitude information T[x,y] 362is representative of a target image (e.g. a photograph). The phaseinformation ψ[x,y] is used to produce a holographic representative ofthe target image at an image plane.

Since the magnitude and phase are intrinsically combined in the Fouriertransform, the transformed magnitude (as well as phase) contains usefulinformation about the accuracy of the calculated data set. Thus, thealgorithm may provided feedback on both the amplitude and the phaseinformation.

The algorithm shown in FIG. 2 can be considered as having a complex waveinput (having amplitude information 301 and phase information 303) and acomplex wave output (also having amplitude information 311 and phaseinformation 313). For the purpose of this description, the amplitude andphase information are considered separately although they areintrinsically combined to form a data set. It should be remembered thatboth the amplitude and phase information are themselves functions of thespatial coordinates x and y and can be considered amplitude and phasedistributions.

Referring to FIG. 2, processing block 350 produces a Fourier transformfrom a first data set having magnitude information 301 and phaseinformation 303. The result is a second data set, having magnitudeinformation and phase information ψ_(n)[x,y] 305. The amplitudeinformation from processing block 350 is set to a distributionrepresentative of the light source but the phase information ψ_(n)[x,y]305 is retained. Phase information 305 is quantised by processing block354 and output as phase information ψ[x,y] 309. Phase information 309 ispassed to processing block 356 and combined with the new magnitude byprocessing block 352. The third data set 307, 309 is applied toprocessing block 356 which performs an inverse Fourier transform. Thisproduces a fourth data set R_(n)[x,y] in the spatial domain havingamplitude information |R_(n)[x,y] 311 and phase information

R_(n)[x,y] 313.

Starting with the fourth data set, its phase information 313 forms thephase information of a fifth data set, applied as the first data set ofthe next iteration 303′. Its amplitude information R_(n)[x,y] 311 ismodified by subtraction from amplitude information T[x,y] 362 from thetarget image to produce an amplitude information 315 set. Scaledamplitude information 315 (scaled by α) is subtracted from targetamplitude information T[x,y] 362 to produce input amplitude informationη[x,y] 301 of the fifth data set for application as first data set tothe next iteration. This is expressed mathematically in the followingequations:

R _(n−1) [x,y]=F′{exp(ψ _(n) [u,v])}

ψ_(n) [u,v]=

F{η·exp(i

R _(n) [x,y])}

θ=T[x,y]−α(|R _(n) [x,y]|−T[x,y])

Where:

-   F′ is the inverse Fourier transform.-   F if the forward Fourier transform.-   R is the replay field.-   T is the target image.-   is the angular information.-   ψ is the quantized version of the angular information.-   ε is the new target magnitude, ε≧0-   α is a gain element ˜1

The gain element α may be selected based on the size and rate of theincoming target image data.

In the absence of phase information from the preceding iteration, inthis example, the first iteration of the algorithm uses a random phasegenerator to supply phase information as a starting point. FIG. 3 showsan example random phase seed.

In an alternative example, the resultant amplitude information fromprocessing block 350 is not discarded. The target amplitude information362 is subtracted from amplitude information to produce new amplitudeinformation. A multiple, typically a multiple less than unity, of thenew amplitude information is subtracted from amplitude information 362to produce the input amplitude information for processing block 356.

Further alternatively, the phase is not fed back in full and only aportion proportion to its change over the last few, for example two,iterations is fed back.

Such techniques have been developed for real-time video projectors inwhich the holograms are calculated in real-time from input video data,for example. However, in examples set out in the present disclosure, itis not essential to calculate the hologram or holograms in real-time.

In examples, the computer-generated holograms are pre-calculated. Afinite number of predetermined computer-generated holograms is stored inlocal memory. This is possible since only a finite small number ofpredetermined holograms are required by the system.

The skilled person will therefore understand that any technique forcalculating the phase-only hologram is therefore suitable. For example,the hologram may equally be calculated by the techniques known as DirectBinary Search or Simulated Annealing.

However, in the described examples, the holograms are calculated by theGerchberg-Saxton algorithm or a modified version of the Gerchberg-Saxtonalgorithm.

The present disclosure relates to using computer-generated holograms toprovide a controllable light distribution for an illumination devicesuch as a headlamp. The holograms are stored in memory and retrieved asnecessary. However the disclosure also envisages calculation inreal-time.

Known headlamps either have moving (motorised) optics or assemblies thatallow the light beam to move left or right and up and down. However,such systems are extremely limiting.

FIG. 4 shows an example in which a holographic reconstruction isprojected by projection optics onto, for example, a road.

FIG. 4 shows an illumination system (300) having an SLM-based system(305) for providing a real image of a holographic reconstruction (310).The SLM-based system (305) corresponds to FIG. 1. The holographicreconstruction (310) is formed at a so-called replay field. Theholographic reconstruction is projected by a projection lens (420) on toa road, for example.

The skilled person will understand that any suitable projection opticsmay be used and the present disclosure is not limited to a projectionlens. The projection optics may equally be a mirror, for example.

Some aspects of the optical system may be incorporated into the hologramto simplify the projection system or to aid beam conditioning.

There is therefore provided a controllable illumination device, such asa headlamp. The device can produce a light distribution, on a road forexample, by projecting a holographic reconstruction.

The light distribution, or projected light, may be referred to as a“beam”.

It can be understood that the projected light distribution may be moved(steered) by moving the holographic reconstruction. The image (e.g. ablock of white light) may be moved from one part of the replay field toanother part of the replay field. The image may be moved or nudged inany chosen direction. The skilled person will understand how tocalibrate the system such that known holograms (and therefore knownreplay fields) correspond to known illumination patterns projected onthe road. Accordingly, it can be understood that the position of thebeam may be controllably adjusted.

The skilled person will understand that the headlamps of a vehicle maybe operated in a so-called high (or full) beam mode, or any other modeas may be required by the driver.

In an example, there is provided a headlamp having an adjustable beamshape. This is achieved by changing the hologram—more specifically, bychanging the phase delay distribution represented on/written to thespatial light modulator. In turn, this changes the holographicreconstruction (for example, the distribution of reconstructed light)and therefore the projected light distribution (or beam). In accordancewith these examples, with high beam on as a car approaches, the hologramis altered to put a black out region for the oncoming driver, preventinghim/her from becoming dazzled. As soon as the car has passed the fullbeam is restored. In some examples the beam shape is adjusteddynamically.

In other examples, the colour temperature or even the colour of the beamitself is dynamically altered according to the weather/day/night timeconditions. Moreover, the colour distribution within the beam may alsobe controlled.

In further examples, the distribution of the beam can be alteredaccording to road conditions, giving better visibility of the verge etc.This could be under driver control.

In examples, the beam can also be steered, left/right (for example, inresponse to continental/UK beam requirements), dynamically when drivinground corners. The beam can also be steered up and down according toroad conditions/car loading.

Advantageously, the projection optics (for example, a mirror assembly)required for examples disclosed may be cheaper to produce as a result ofhaving dynamic control over the light distribution.

In examples, vehicle indicators are included as part of thefunctionality, by directing orange (red/green) light to one side of theassembly, for example. Again the hologram can do this dynamically. Thisside of the assembly may or may not use a diffuser to widen the lightdistribution angle.

In examples, the colour of the illumination in different areas of thescene is selectively altered depending upon road conditions or drivererror, for example. In examples, one side of the scene is red (or anyother distinguishing colour) if the driver strays over the centre of theroad, for example. In examples, the indicators are presenteddynamically. In further examples, street furniture or signs, forexample, may be selectively and dynamically illuminated.

Further advantageously, in examples using a pixellated spatial lightmodulator, because of the pixellation, the higher diffraction orders(which are normally unwanted in video imaging systems, for example)could be used to create the wide angle illumination of the road.

Advantageously, as this system need not have moving parts, it can bemore robust than the current systems.

In further examples, the illumination may contain infra-red (IR) lightalso under holographic control, which may be used, for example, as partof a larger night vision system or head-up display. In examples, thereis provided a holographic projection of an IR grid. Moreover, the IRsystem could, optionally, be used as part of a forward looking system tojudge road quality (pot holes etc) allowing the suspension to adjust andallowing the headlight to compensate accordingly. The skilled personwill understand how the suspension and headlight, for example, may beadjusted in response to measurements or detections made by the IRsystem.

In examples, the light distribution is linked to GPS (satellitenavigation) to predict the lighting needs of the driver.

In one example, there is provided an auto-dimming headlamp.

Auto dipping or auto anti dazzle headlamps are already a feature on somevehicles, however the implementation is very simple: a forward lookinglight sensor detects the presence of oncoming main headlights. Once thelight level reaches a certain level the main beams are automaticallyswitched to dipped beams or a mechanical shutter is actuated obscuringthe part of the light beam which would otherwise dazzle the driver ofthe oncoming vehicle. The mechanical shutter or “beam obscurer”physically blocks part of the light beam.

In an embodiment, the lighting device in accordance with the presentdisclosure provides a more intelligent auto-dipping or auto anti-dazzleheadlamp. In an embodiment, a forward looking camera is used torecognise that a vehicle is approaching. However, the skilled personwill understand that other techniques for recognising that a vehicle isapproaching may be equally suitable. Even at great distance, the systemin accordance with the present disclosure can start to alter the beamdistribution (that is, the pattern of illumination) in view of theapproaching vehicle. At a large distance, only a small amount of thebeam would need to be redirected but, as the vehicle approached, thesystem tracks the vehicle and, in response, changes the pattern ofillumination. The pattern of illumination may change in size and/shapein response to the approaching vehicle. The skilled person willunderstand how the pattern of illumination may be altered to reduce theimpact of the pattern of illumination (e.g. reduce “dazzle”) on thedriver on an on-coming vehicle, for example.

As the lighting device in accordance with the present disclosure isbased upon phase-only holographic techniques (i.e. a selectable phasedelay distribution is applied to form phase-modulated light in theFourier domain) light is not wastefully blocked during auto-dimming orauto anti-dazzle. Instead, a hologram (phase delay distribution pattern)is calculated and written to the phase modulator which, whenilluminated, creates a light distribution which has the desireddistribution (i.e. pattern of illumination). That is, for the sake ofclarity, during the hologram computation process, light otherwise in the“obscured” area is redirected in to other areas of the light beamdistribution. In this way, light is not simply blocked and lost from thesystem—as with a mechanical shutter system—but is instead directed toother parts of the pattern of illumination. Accordingly, the system ismore energy efficient.

FIG. 6 shows an algorithm, in accordance with an embodiment, fordynamically modifying the head lamp illumination profile in view of anapproaching vehicle.

In FIG. 6, there is shown a camera 601 arranged to monitor forapproaching vehicles. As block 603, a determination is made as towhether an approaching vehicle has been detected. For example, block 603may determine if the image of a headlamp (of an approaching vehicle)appears in a detected frame of a sequence of image frames from camera601. If an approaching vehicle is not detected, “ordinary” light beamdata is used to form hologram (phase delay distribution) 713. The“ordinary” light beam data 611 provides the pattern of illuminationsuitable for when an approaching vehicle is not present (i.e. noobscured area or areas). However, if an approaching vehicle is detected,the process moves to block 605 in which a calculation of the location ofthe approaching vehicle is made. The skilled person will understand thata variety of techniques may be used to determine the approximatelocation or proximity of an approaching vehicle from an image frame. Theterm “obsuration area” may be used to refer to the area which should nolonger receive light from the headlamp (i.e. no longer be part of thearea illuminated) because of the proximity of the approaching vehicle.In prior art methods, the “obsuration area” is the part of the lightbeam that would be blocked by a mechanical shutter. At block 607, adetermination is made as to the size of the “obsuration area” is madebased, for example, on the separation of the headlamps of theapproaching vehicle. The skilled person will understand that, in otherembodiments, other measurements related to the approaching vehicle maybe used to determine the size of the “obsuration area”. At block 609,the obsuration area is applied (such as added) to “ordinary” light beamdata 611 to form a modified hologram 613 (phase delay distribution). Atblock 617, the phase delay distribution is applied (such as written) tothe addressable spatial light modulator. The method may repeat fromblock 603.

There is therefore provided a method of modifying the illumination of ascene, the method comprising calculating the approximate proximity of anapproaching vehicle, determining the size and shape of an obsurationarea, modifying the set of phase delays on a spatial light modulator,illuminating the spatial light modulator to provide an exit beam,applying the exit beam to Fourier optics to form an image and projectingthe image to provide a modified scene-illuminating beam.

In another embodiment, the lighting device in accordance with thepresent disclosure provides an auto-highlighting illumination system.FIG. 7 shows an example in which the headlamps 701 and 703 of a vehicle709 are used to illuminate the road 705 and, additionally, a road sign707. In addition to the ability to redirect light to prevent dazzlingthe drivers of oncoming vehicles (obscuration), in an embodiment, thelighting device in accordance with the present disclosure is used todynamically provide additionally illumination to help driver awarenessof the driving environment.

Many new vehicles come fitted with road sign recognition and collisionavoidance radar systems, both of these can serve as an input to theholographic calculation engine of the present disclosure. In anembodiment, the output of these systems is used so that it not onlyindicates what has been found, but also where in space the object can befound (that is, its proximity relative to the vehicle). The holograms(i.e. the phase delay distributions applied to the spatial lightmodulator) are then calculated to redirect a programmable amount oflight to the specified location thereby drawing the driver's attentionto the obstacle or road side information.

FIG. 8 shows a flow diagram of how this is achieved in one embodiment.At block 805, a determination is made as to whether input is receivedfrom a road sign recognition system 801 and/or a collision avoidancesystem 803. If input is not received at block 805, the scene isilluminated by an “ordinary” pattern of illumination. That is,“ordinary” light beam data is used to form the hologram (i.e. phasedelay distribution) at block 813. If input is received at block 805, adetermination is made at block 807 as to the size and distribution ofthe required additional illumination. The skilled person will understandthat data from the road sign recognition system 801 and/or a collisionavoidance system 803 may be used to determine which parts of the sceneshould be additional illuminated in accordance with the presentdisclosure. Data relating to the required additional illumination iscombined (such as added) with the “ordinary” light beam data to formhologram (phase delay distribution) at block 813. The hologram isapplied (such as written) to the spatial light modulator which is thenilluminated to form a pattern of illumination.

There is therefore provided a method of modifying the illumination of ascene, the method comprising calculating the approximate proximity of afeature of interest in the scene, determining the size and shape of anarea for additional illumination, modifying the set of phase delays on aspatial light modulator, illuminating the spatial light modulator toprovide an exit beam, applying the exit beam to Fourier optics to forman image and projecting the image to provide a modifiedscene-illuminating beam.

In another embodiment, there is provided an improved system for dynamicroad illumination.

In conventional projection style head lamps, there is a legalrequirement for the light to dynamically adjust to prevent dazzlingother road users. If the vehicle is tilted either forwards or backwards,a sensor is used to determine the extent of the tilt and motorisedactuators alter the projection lens positions to correct thebeam-pointing error. Additionally there may be a need to move fromdriving on the left to driving on the right. In such cases, the beamdistribution and/or direction may need to be altered. Some modernvehicles have additional motorised actuators to enable such features. Inembodiments, the lighting device in accordance with the presentdisclosure is used to provide such control.

The holographic system of the present disclosure has the same capabilitybut with the advantage that no moving parts are required. Thecomputation has the ability to control the direction and distribution ofthe light beam there by offering the ability to correct the roadillumination.

In further embodiments, this functionality is extended, if the vehiclehas navigation information about the geography of the road ahead, thiscould be fed in to the computation system thereby improving the roadillumination for the driver.

A specific example of dynamic headlamps is where the headlamps of thevehicle steer left or right when cornering. Traditionally, this has beenachieved by a mechanical linkage between the headlamps and the steeringcolumn, however this could equally be achieved by rotational sensing ofthe steering wheel and motorised control of the lights/lenses.

In embodiments, the holographic system of the present disclosureachieves the same effect: a sensor detects the rotation of the steeringwheel which alters the phase delay distribution and therefore thepattern of illumination. In further embodiments, this is furtherenhanced by interfacing the vehicles' GPS navigation system to theholographic illumination computer. The GPS unit indicates that a bend isapproaching and the holographic system allocates additional light to theapproach of the corner, thereby illuminating the bend in the road beforesteering input has been applied.

In a further embodiment, there is provided an infra-red (IR) holographicillumination system.

In addition to the benefits that dynamic holographic illumination bringsthe driver for awareness of his surroundings, in an embodiment, theholographic system of the present disclosure is used to generateinvisible infra-red holographic light distributions that would enablethe vehicle to detect the condition of the road ahead.

In an embodiment, the system projects an IR pattern of illuminationcomprising a grid pattern on to the road ahead of the vehicle. Aforward-looking IR camera detects the holographic IR grid. Where potholes, road cambers etc exist, the grid would be distorted and thisdistortion is detected by a camera, for example. This enables thevehicle to alter its suspension settings and power distribution toenable optimal safety & comfort.

FIG. 9 shows a first example grid 901 on a smooth road and a secondexample grid 903 on a road with pot holes, such as pot hole 905.

There is therefore provided a method of projecting an infra-red grid ona scene, the method comprising forming a variable set of phase delays ona spatial light modulator, illuminating the spatial light modulator withinfra-red light to provide an exit beam, applying the exit beam toFourier optics to form an infra-red image in the spatial domaincomprising a grid pattern and projecting the infra-red image onto thescene. The method may further comprise capturing an image of scene andcomparing the captured image with the projected infra-red image todetect abnormalities in the scene.

In another embodiment, the same IR is used as part of a Light Detectionand Ranging (LIDAR) collision avoidance system. The IR wavelength ischosen such that it has greater ability to penetrate fog (reduced waterabsorption) there by allowing the vehicle to have greater sensing rangeeven in reduced visibility conditions. In these embodiments, the patternof illumination is therefore an infra-red pattern of illumination andthe device further comprises detection means for detecting the patternof illumination on the scene and processing means for identifyingabnormalities in the scene by, for example, identifying differencesbetween the detected pattern of illumination and pattern of illuminationcorresponding to the phase delay distribution.

As the phase only holographic system is inherently diffractive innature, any white light source would be distributed differently as afunction of its wavelength (the longer the wavelength the larger thediffraction angle). Therefore, in an embodiment, the holographic systemis used to correct this chromatic variation. In further embodiments, thesystem is also used to manipulate the ratios of the wavelengths (therebyaltering the colour of the light) either for the entire beam or forselective portions of the beam.

For example it may be better when driving in fog to have a headlamp witha yellow hue, the holographic system of the present disclosure could beused to dynamically attenuate the blue content of the white lightsource.

In yet further embodiments, this concept is further extended, so thatthe main head lamps act as indicators. In a small portion of the mainbeam, the wavelength distribution alternates between, for example,orange and white by dynamically attenuating the blue & green wavelengthsin the specified region of the main beam. Given that all theseparameters are under software control if affords the designer of theillumination system great flexibility and power to optimise the totalillumination output for all driving conditions.

Unlike video projection systems, for instance, examples of the presentdisclosure do not require the holographic reconstruction or projectedimage to be a high quality image. The light source for the spatial lightmodulator therefore only needs to be at least partially coherent. Forexample, in examples, the light source may therefore comprise at leastone light emitting diode. In examples, the light source may comprise atleast one laser. The skilled person will understand that other lightsources may be equally suitable.

The spatial light modulator may be transmissive or reflective. That is,the phase modulated light may be output from the spatial light modulatorin transmission or reflection.

The holographic reconstruction (or image) may be affect by the so-calledzero order problem which is a consequence of the diffractive nature ofthe reconstruction. Such zero-order light is usually regarded as “noise”and includes for example specularly reflected light, and other unwantedlight from the SLM.

This “noise” is generally focussed at the focal point of the Fourierlens, leading to a bright spot at the centre of a reconstructedhologram. Conventionally, the zero order light is simply blocked outhowever this would clearly mean replacing the bright spot with a darkspot.

However, in examples, the zero order (or DC spot, which is also normallyunwanted) is used to contribute to the centre of the light beam. Forexample, zero order may be advantageously used to provide greaterillumination at the centre of the beam.

As the hologram contains three dimensional information, it is possibleto displace the reconstructed hologram into a different plane inspace—see, for example, published PCT application WO 2007/131649incorporated herein by reference.

Whilst examples described herein relate to displaying one hologram perframe, the present disclosure is by no means limited in this respect andmore than one hologram may be displayed on the SLM at any one time.

For example, examples implement the technique of “tiling”, in which thesurface area of the SLM is further divided up into a number of tiles,each of which is set in a phase distribution similar or identical tothat of the original tile. Each tile is therefore of a smaller surfacearea than if the whole allocated area of the SLM were used as one largephase pattern. The smaller the number of frequency component in thetile, the further apart the reconstructed pixels are separated when theimage is produced. The image is created within the zero diffractionorder, and it is preferred that the first and subsequent orders aredisplaced far enough so as not to overlap with the image and may beblocked by way of a spatial filter.

As mentioned above, the image produced by this method (whether withtiling or without) comprises spots that form image pixels. The higherthe number of tiles used, the smaller these spots become. Taking theexample of a Fourier transform of an infinite sine wave, a singlefrequency is produced. This is the optimum output. In practice, if justone tile is used, this corresponds to an input of a single phase of asine wave, with a zero values extending in the positive and negativedirections from the end nodes of the sine wave to infinity. Instead of asingle frequency being produced from its Fourier transform, theprinciple frequency component is produced with a series of adjacentfrequency components on either side of it. The use of tiling reduces themagnitude of these adjacent frequency components and as a direct resultof this, less interference (constructive or destructive) occurs betweenadjacent image pixels, thereby improving the image quality.

Preferably, each tile is a whole tile, although it is possible to usefractions of a tile.

The present disclosure is not limited to monochromatic projection orillumination.

A colour 2D holographic reconstruction can be produced and there are twomain methods of achieving this. One of these methods is known as“frame-sequential colour” (FSC). In an FSC system, three lasers are used(red, green and blue) and each laser is fired in succession at the SLMto produce a composite colour reconstruction. The colours are cycled(red, green, blue, red, green, blue, etc.) at a fast enough rate suchthat a human viewer sees a polychromatic image from a combination of thethree lasers. Each hologram (phase delay distribution on the spatiallight modulator) is therefore colour-specific. For example, the first“frame” may be produced by firing the red laser for 1/75^(th) of asecond, then the green laser would then be fired for 1/75^(th) of asecond, and finally the blue laser would be fired for 1/75^(th) of asecond.

An alternative method, that will be referred to as “spatially separatedcolours” (SSC) involves all three lasers being fired at the same time,but taking different optical paths, e.g each using a different SLM, andthen combining to form the colour image.

An advantage of the frame-sequential colour (FSC) method is that thewhole SLM is used for each colour. This means that the quality of thethree colour images produced will not be compromised because all pixelson the SLM are used for each of the colour images. However, adisadvantage of the FSC method is that the overall image produced willnot be as bright as a corresponding image produced by the SSC method bya factor of about 3, because each laser is only used for a third of thetime. This drawback could potentially be addressed by overdriving thelasers, or by using more powerful lasers, but this would require morepower to be used, would involve higher costs and would make the systemless compact.

An advantage of the SSC (spatially separated colours) method is that theimage is brighter due to all three lasers being fired at the same time.However, if due to space limitations it is required to use only one SLM,the surface area of the SLM can be divided into three equal parts,acting in effect as three separate SLMs. The drawback of this is thatthe quality of each single-colour image is decreased, due to thedecrease of SLM surface area available for each monochromatic image. Thequality of the polychromatic image is therefore decreased accordingly.The decrease of SLM surface area available means that fewer pixels onthe SLM can be used, thus reducing the quality of the image. The qualityof the image is reduced because its resolution is reduced.

As can be understood from the foregoing, the light source may compriseat least one infra-red light source, for example.

In examples, the spatial light modulator is a reflective Liquid Crystalover Silicon (LCOS) device. LCOS devices are a hybrid of traditionaltransmissive liquid crystal display devices, where the front substrateis glass coated with Indium Tin Oxide to act as a common electricalconductor. The lower substrate is created using a silicon semiconductorprocess with an additional final aluminium evaporative process beingused to create a mirrored surface, these mirrors then act as the pixelcounter electrode. Such SLMs can be formed to have a fill factor ofbetter than 90 per cent.

LCOS devices are now available with pixels between 4.5 μm and 12 μm. Thenecessary size is determined by the application to which the sLM is tobe put, the mode of operation and therefore the amount of circuitry thatis required at each pixel.

The structure of an LCOS device is shown in FIG. 5.

A LCOS device is formed using a single crystal silicon substrate (402).It has a 2D array of square planar aluminium electrodes (401), spacedapart by gaps (401 a), arranged on the upper surface of the substrate.Each of the electrodes (401) is connected to circuitry (402 a) buried inthe substrate (402) to allow addressing of each electrode. Each of theelectrodes forms a respective planar mirror. An alignment layer (403) isdisposed on the array of electrodes, and a liquid crystal layer (404) isdisposed on the alignment layer (403). A second alignment layer (405) isdisposed on the liquid crystal layer (404) and a planar transparentlayer (406), e.g. of glass, is disposed on the second alignment layer(405). A single transparent electrode (407) e.g. of ITO is disposedbetween the transparent layer (406) and the second alignment layer(405).

Each of the square electrodes (401) defines, together with the overlyingregion of the transparent electrode (407) and the intervening liquidcrystal material, a controllable phase-modulating element (404), oftenreferred to as a pixel. The effective pixel area, or fill factor, is thepercentage of the total pixel which is optically active, taking intoaccount the space between pixels (401 a). By control of the voltageapplied to each electrode (401) with respect to the transparentelectrode (407), the properties of the liquid crystal material of therespective phase modulating element may be varied, thereby to provide avariable delay to light incident thereon. The effect is to providephase-only modulation to the wavefront, i.e. no amplitude effect occurs.

A major advantage of using a reflective LCOS spatial light modulator isthat the liquid crystal layer is half the thickness that it would be ifa transmissive device were used. This greatly improves the switchingspeed of the liquid crystal A LCOS device is also uniquely capable ofdisplaying large arrays of phase only elements in a small aperture.Small elements (typically approximately 10 microns) result in apractical diffraction angle (a few degrees).

It is easier to adequately illuminate the small aperture (a few squarecentimetres) of a LCOS SLM than it would be for the aperture of a largerliquid crystal device. LCOS SLMs also have a large aperture ratio, thereis very little dead space between the pixels (as the circuitry to drivethem is buried under the mirrors). This is an important issue tolowering the optical noise in the replay field.

The above device typically operates within a temperature range of 10° C.to around 50° C.

As a LCOS device has the control electronics embedded in the siliconbackplane, the Fill factor of the pixels is higher, leading to lessunscattered light leaving the device.

Using a silicon backplane has the advantage that the pixels areoptically flat, which is important for a phase modulating device.

Whilst examples relate to a reflective LCOS SLM, the skilled person willunderstand that any SLM can be used including transmissive SLMs.

The disclosure has been put in the context of vehicle headlamps but isof course applicable to other devices for lighting purposes, such assearch lights, torches and the like.

The invention is not restricted to the described examples but extends tothe full scope of the appended claims.

1. A light detection and ranging “LIDAR” collision avoidance system, the system comprising a lighting device arranged to produce a controllable light beam for illuminating a scene, the device comprising: an addressable spatial light modulator arranged to provide a selectable phase delay distribution to a beam of incident light; Fourier optics arranged to receive phase-modulated light from the spatial light modulator and form a light distribution; and projection optics arranged to project the light distribution to form a pattern of illumination as said controllable light.
 2. The LIDAR system of claim 1, wherein the light is infrared light.
 3. The LIDAR system of claim 1, wherein the pattern of illumination comprises a grid pattern.
 4. The LIDAR system of claim 1, further comprising a forward-looking camera arranged to capture an image of the scene.
 5. The LIDAR system of claim 1, wherein the system is arranged to compare the captured image with the projected pattern of illumination.
 6. The LIDAR system of claim 1, wherein the system is arranged to identify differences between the captured image and the projected pattern of illumination.
 7. The LIDAR system of claim 1, wherein the processor is arranged to detect an abnormality in the scene.
 8. The LIDAR system of claim 1, wherein the system is arranged to select the phase delay distribution to steer the controllable light beam.
 9. The LIDAR system of claim 1, wherein the system is arranged to select the phase delay distribution to illuminate chosen areas of the scene while not illuminating other areas.
 10. The LIDAR system of claim 1, wherein the system is arranged to reading the phase delay distribution from a memory.
 11. The LIDAR system of claim 1, wherein the system is arranged to detect the approximate proximity of a vehicle and modify the selectable phase delay distribution on the spatial light modulator to provide a modified scene-illuminating beam.
 12. The LIDAR system of claim 1, wherein the phase delay distribution comprises a representation of the pattern of illumination in the Fourier domain.
 13. The LIDAR system of claim 1, wherein the Fourier optic is arranged to perform a time-space transformation.
 14. The LIDAR system of claim 1, wherein the spatial light modulator is liquid crystal on silicon device.
 15. A vehicle comprising the LIDAR system of claim
 1. 